Nuclear reactors create energy by nuclear fission, a process that splits the nucleus of an atom in two. An element with a heavy nucleus, most commonly uranium-235 (U-235), is hit by a free neutron. This divides the heavy nucleus into two lighter nuclei. The action of this split releases energy in the form of heat and radiation. Two to three other neutrons are released during this process. These neutrons then hit more U-235 nuclei and split them. This reaction continues on and on, forming what is called a chain reaction.
Chain reaction
Drawings courtesy of atomicarchive.com.
Fission reaction diagram.
Fission reactions fall into three categories, based on the speed of the reaction and the energy released. A reaction is subcritical, when on average, less than one of the free neutrons from each fission reaction hits another U-235 atom. This means that the fission reaction is below critical mass and will eventually die out. In a supercritical reaction, on average more than one of the neutrons hits another U-235 atom. Because of the heat produced by this rapid fission, the whole reaction explodes. An atomic bomb is a supercritical reaction.
For power generation, a critical reaction—the most stable type of reaction—must occur. This happens when on average exactly one of the neutrons from each fission reaction hits another U-235 atom. The fuel is at critical mass. It maintains a stable temperature, not explosively hot, and the reaction is not too slow to maintain fission in the fuel. The heat from the nuclear reaction converts water to steam. The steam turns turbines attached to generators, creating electricity. In a nuclear reactor, the reaction stays at critical mass through the use of devices to speed up or slow down the reaction as needed.
What makes a nuclear reaction so useful for generating power is that it releases much more energy than the typical combustion reaction. For example, a coal plant produces about 1 kilowatt of electricity by burning 0.45 kg (1 lb) of coal. The fission of 0.45 kg of uranium produces about 3 million kilowatts.